Quorum-sensing is the regulation of bacterial gene expression in response to change in cell density. Bacteria that utilize quorum-sensing signaling pathways synthesize signaling molecules (auto-inducers), which have been found in nature as N-homoserine lactones or small peptides (Miller and Bassler. Ann. Rev. Microbiol. 2001; 55:165-169). Auto-inducer levels are directly proportional to the size of the bacterial population (Fuqua, Parsek and Greenberg. Annu. Rev. Gen. 2002; 3:685-695), and at threshold levels, as detectable by bacterial cell receptors, auto-inducer binding alters bacterial gene expression (Miller and Bassler, supra). Quorum-sensing-based regulation of gene expression is critical for the pathogenesis of clinically important bacterial infections, such as those due to Pseudomonas aeruginosa in patients with cystic fibrosis (Erickson et al. Infect. Immun. 2001; 7:1783-1790), or Vibrio cholerae (Miller et al. Cell. 2002; 110:303-314).
Quorum-sensing has been well-characterized in Vibrio harveyi, bioluminescent bacteria that freely live in the ocean floor sediment or on the exterior of fish (Ramaiah et al. J. Appl. Microbiol. 2002; 93:108-116). The luminescence genes are expressed only when the V. harveyi populations are at high cell density, under the control of the lux quorum-sensing system. The luxCDABE operon, whose expressed polypeptides confer luminescence, is regulated by signaling pathways that are stimulated by the auto-inducer ligands, AI-1 (AHL) and AI-2. Synthesis of AI-1 requires the product of the luxM gene. AI-1 diffuses freely through the cell wall into the extracellular milieu, and when sufficient quantities are recognized by its sensor histidine kinase, LuxN, a hybrid two-component system-signaling cascade is initiated (Miller and Bassler, supra).
The Vibrio harveyi lux cascade also is regulated by another auto-inducer molecule, AI-2, which is predicted to be a furanosyl borate diester and synthesized by the product of the luxS gene. The luxS gene product converts S-ribosylhomocysteine to 4,5-dihydroxyl-2,3-pentanedione (DPD), catalyzing AI-2 formation (Chen et al. Nature 2001; 415:545-549). V. harveyi strain BB170, in which luxN is mutated, is unable to detect AI-1 molecules, and may be used to detect AI-2 or AI-2-like molecules in its milieu (Bassler, Wright and Silverman. Mol. Microbiol. 1994; 13:273-286).